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Abstract:

A fuel cell system includes an exhaust pipe through which anode purge gas
is discharged into the atmosphere, a heat medium passage through which
heat medium is flowed, a circulation pump for circulating the heat medium
through the heat medium passage, a heat exchanger provided in the heat
medium passage, a heat exchanger fan for generating airflow through the
heat exchanger, and a guide by which the airflow is guided so as to
diffuse the anode purge gas. The circulation pump and the heat exchanger
fan are operated during anode purge in such a way that the heat exchanger
fan is operated regardless of the heat medium temperature, and a flow
rate of the heat medium flowing through the heat medium passage is zero
or lower than a flow rate when output of a fuel cell is maximum.

Claims:

1. A fuel cell system for generating electric power through a reaction
between oxygen supplied to a cathode and hydrogen supplied to an anode of
a fuel cell, comprising:an exhaust pipe through which anode purge gas
from the fuel cell is discharged into the atmosphere;a heat medium
passage through which heat medium is flowed, the heat medium passage
being thermally coupled to the fuel cell;a circulation pump for
circulating the heat medium through the heat medium passage;a heat
exchanger provided in the heat medium passage;a heat exchanger fan for
generating airflow through the heat exchanger;a guide by which the
airflow is guided so as to diffuse the anode purge gas discharged from
the exhaust pipe; anda controller for controlling the operation of the
circulation pump and the heat exchanger fan,wherein the circulation pump
and the heat exchanger fan are operated under the control of the
controller during anode purge for a first time after the start of the
fuel cell in such a way that the heat exchanger fan is operated
regardless of the heat medium temperature, and a flow rate of the heat
medium flowing through the heat medium passage is zero or lower than a
flow rate when output of the fuel cell is maximum.

2. The fuel cell system according to claim 1, wherein the heat exchanger
fan is operated during the anode purge at the maximum rotation speed
within its speed control range.

3. The fuel cell system according to claim 1, further comprising a diluter
for diluting anode purge gas from the fuel cell with cathode off-gas from
the cathode of the fuel cell.

4. The fuel cell system according to claim 3, wherein the first time is
shorter than a second time after which controlling of the heat exchanger
fan based on the heat medium temperature is started.

5. The fuel cell system according to claim 1, further comprising a purge
valve through which anode purge gas from the fuel cell is discharged, the
purge valve being opened and closed intermittently during the control of
the anode purge.

6. The fuel cell system according to claim 5, wherein the heat exchanger
fan is operated regardless of the heat medium temperature and the flow
rate of the heat medium is lowered, while the purge valve is
intermittently opened.

7. A method of controlling a fuel cell system for generating electric
power through a reaction between oxygen supplied to a cathode and
hydrogen supplied to an anode of a fuel cell, the fuel cell system
including:an exhaust pipe through which anode purge gas from the fuel
cell is discharged into the atmosphere;a heat medium passage through
which heat medium is flowed, the heat medium passage being thermally
coupled to the fuel cell;a circulation pump for circulating the heat
medium through the heat medium passage;a heat exchanger provided in the
heat medium passage;a heat exchanger fan for generating airflow through
the heat exchanger; anda guide by which the airflow is guided so as to
diffuse the anode purge gas discharged from the exhaust pipe,the method
comprising the steps of:performing anode purge during a given time after
the start of the fuel cell;operating the heat exchanger fan during the
anode purge regardless of the heat medium temperature; andoperating the
circulation pump during the anode purge so that a flow rate of the heat
medium flowing through the heat medium passage is zero or lower than a
flow rate when output of the fuel cell is maximum.

Description:

BACKGROUND OF THE INVENTION

[0001]The present invention relates to a fuel cell system, and more
particularly to a fuel cell system and method of controlling the same
that allows a reduced hydrogen concentration in anode purge gas
discharged during anode purge at the start of the fuel cell system.

[0002]As a power source that helps to prevent global warming due to
exhaust gas, there has been developed a fuel cell that produces electric
energy through a chemical reaction between hydrogen and oxygen. In a
system with a fuel cell, generally, the cathode is not directly supplied
with oxygen but supplied with air containing oxygen. In such fuel cell
system, part of the water produced in the cathode and part of the
nitrogen contained in the air supplied to the cathode are diffused back
through the electrolyte membrane from cathode to anode. Thus, when the
fuel cell is started after an interval of time in hours or days, or when
the fuel cell is continued to operate, the concentration of hydrogen as
anode gas may be decreased relatively due to the increased concentration
of water and nitrogen in the anode, which may result in reduced
efficiency of electric power generation. To prevent this, generally,
purging of the anode (hereinafter referred to as anode purge) is carried
out by discharging water and nitrogen in the anode with hydrogen as anode
gas. In such anode purge, anode purge gas is diluted with cathode off-gas
or using a diluter before being discharged into the atmosphere so that
hydrogen concentration in the anode purge gas is decreased to a safe
level.

[0003]In a conventional fuel sell system disclosed in Japanese Unexamined
Patent Application Publication No. 2006-99989, a hydrogen exhaust
passage, a hydrogen purge valve and a hydrogen dilution fan are provided
on the anode outlet side. In the anode purge when the fuel cell is
started or stopped, the hydrogen purge valve is intermittently opened for
a short time, and the hydrogen dilution fan is operated depending on the
hydrogen concentration in the anode purge gas discharged from the
hydrogen exhaust passage so as to diffuse and dilute the anode purge gas
with air. As a result, the hydrogen concentration in the anode purge gas
is decreased below a predetermined level.

[0004]In another conventional fuel cell system disclosed in Japanese
Unexamined Patent Application Publication No. 2008-235205, anode purge
gas is discharged into the atmosphere through a diluter and an exhaust
pipe provided on the anode outlet side. The diluter is connected to a
cathode off-gas passage through which cathode off-gas (air) is supplied
to the diluter. The fuel cell system further includes a heat medium
passage through which cooling water for maintaining the fuel cell at an
appropriate temperature is circulated, a radiator provided in the heat
medium passage, and a radiator fan for blowing air through the radiator.
Airflow generated by the radiator fan is delivered to a position adjacent
to the outlet of the exhaust pipe. Thus, the airflow from the radiator
fan forces the anode purge gas diluted with the cathode off-gas and
discharged from the exhaust pipe into the atmosphere to be diffused,
resulting in reduction of hydrogen concentration in the anode purge gas
below a predetermined level.

[0005]In the fuel cell system disclosed in the publication No. 2006-99989,
however, the hydrogen dilution fan additionally provided results in
increased size of the system and increased number of parts of the system.
On the other hand, in the fuel cell system disclosed in the publication
No. 2008-235205, though the use of existing radiator fan helps to prevent
increased size of the system and increased number of parts of the system,
diffusion of anode purge gas into the atmosphere may not be accomplished
sufficiently in the anode purge at the start of the fuel cell. This is
because, in general, the radiator fan is started to operate when the
cooling water reaches a given temperature and, therefore, the radiator
fan is at a stop or operated at a low speed in a condition that only a
short time has elapsed after the start of the fuel cell and the amount of
heat generated in the fuel cell is still small. In addition, the cathode
off-gas (air) for diluting anode purge gas is not supplied sufficiently
just after the start of the fuel cell, which may result in insufficient
dilution of anode purge gas with the cathode off-gas.

[0006]The present invention is directed to providing a fuel cell system
that allows hydrogen concentration in anode purge gas to be reduced
efficiently and sufficiently during anode purge at a start of the fuel
cell system.

SUMMARY OF THE INVENTION

[0007]In accordance with an aspect of the present invention, a fuel cell
system for generating electric power through a reaction between oxygen
supplied to a cathode and hydrogen supplied to an anode of a fuel cell
includes an exhaust pipe through which anode purge gas from the fuel cell
is discharged into the atmosphere, a heat medium passage through which
heat medium is flowed, the heat medium passage being thermally coupled to
the fuel cell, a circulation pump for circulating the heat medium through
the heat medium passage, a heat exchanger provided in the heat medium
passage, a heat exchanger fan for generating airflow through the heat
exchanger, a guide by which the airflow is guided so as to diffuse the
anode purge gas discharged from the exhaust pipe, and a controller for
controlling the operation of the circulation pump and the heat exchanger
fan. The circulation pump and the heat exchanger fan are operated under
the control of the controller during anode purge for a first time after
the start of the fuel cell in such a way that the heat exchanger fan is
operated regardless of the heat medium temperature, and a flow rate of
the heat medium flowing through the heat medium passage is zero or lower
than a flow rate when output of the fuel cell is maximum.

[0008]In accordance with another aspect of the present invention, a method
of controlling a fuel cell system for generating electric power through a
reaction between oxygen supplied to a cathode and hydrogen supplied to an
anode of a fuel cell is provided. The fuel cell system includes an
exhaust pipe through which anode purge gas from the fuel cell is
discharged into the atmosphere, a heat medium passage through which heat
medium is flowed, the heat medium passage being thermally coupled to the
fuel cell, a circulation pump for circulating the heat medium through the
heat medium passage, a heat exchanger provided in the heat medium
passage, a heat exchanger fan for generating airflow through the heat
exchanger, and a guide by which the airflow is guided so as to diffuse
the anode purge gas discharged from the exhaust pipe. The method includes
the steps of performing anode purge during a given time after the start
of the fuel cell, operating the heat exchanger fan during the anode purge
regardless of the heat medium temperature, and operating the circulation
pump during the anode purge so that a flow rate of the heat medium
flowing through the heat medium passage is zero or lower than a flow rate
when output of the fuel cell is maximum.

[0009]Other aspects and advantages of the invention will become apparent
from the following description, taken in conjunction with the
accompanying drawings, illustrating by way of example the principles of
the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a side view of a forklift truck equipped with a fuel cell
system according to an embodiment of the present invention;

[0011]FIG. 2 is a block diagram of the fuel cell system of FIG. 1;

[0012]FIG. 3 shows graphs explaining the operation of the fuel cell system
of FIGS. 1 and 2;

[0013]FIG. 4 shows graphs as an example for comparison with the embodiment
of the present invention; and

[0014]FIG. 5 shows graphs explaining the operation of a fuel cell system
according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015]FIG. 1 shows a fuel cell forklift truck 10 equipped with a fuel cell
system 12 according to an embodiment of the present invention. The fuel
cell forklift truck 10 is of a battery replacement type in which a lead
acid battery as a power source for traveling and loading in a
conventional battery forklift truck is replaced by the fuel cell system
12. The existing chassis of the conventional battery forklift truck can
be used as the chassis 13 of the fuel cell forklift truck 10 without
major modification. Specifically, the truck body 16 formed by the frame
14 and the weight 15, the travel motor 17, the loading motor 18, and the
power control unit 19 for controlling the motors 17 and 18 are
substantially the same as those of the conventional battery forklift. The
fuel cell system 12 is provided in a casing 11 that is mounted on the
chassis 13.

[0016]Referring to FIG. 2, the fuel cell system 12 includes a fuel cell
20, a hydrogen tank 21 for supplying hydrogen to the anode of the fuel
cell 20, an air compressor 22 for supplying compressed air to the cathode
of the fuel cell 20, a humidifier 23 for humidifying compressed air from
the air compressor 22, and a diluter 24 for diluting purge gas from the
anode of the fuel cell 20. The fuel cell system 12 further includes a
radiator 25 (heat exchanger) and a radiator fan 26 (heat exchanger fan)
for cooling the fuel cell 20, and a controller 27 (hereinafter referred
to as ECU) for controlling the operation of the fuel cell system 12.

[0017]The fuel cell 20 is provided for example by a polymer electrolyte
fuel cell, generating direct current electric energy (direct current
power) by reacting hydrogen supplied from the hydrogen tank 21 with
oxygen contained in the air supplied from the air compressor 22. The
hydrogen tank 21 as a hydrogen source is connected through a pipe 28 to
the hydrogen inlet port (not shown) of the fuel cell 20. The pipe 28 is
provided with a pressure-regulating valve (not shown) by which the
pressure of hydrogen to be supplied from the hydrogen tank 21 to the fuel
cell 20 is reduced to a given level.

[0018]The air compressor 22 is connected to the humidifier 23 through a
pipe 29. The humidifier 23 is connected to the oxygen inlet port of the
fuel cell 20 (not shown) through a pipe 30 and to the off-gas outlet port
(not shown) of the fuel cell 20 through a pipe 31. Compressed air from
the air compressor 22 is humidified in the humidifier 23 and then
supplied through the pipe 30 to the oxygen inlet port of the fuel cell
20. Off-gas from the cathode of the fuel cell 20 is discharged through
the pipe 31 into the humidifier 23.

[0019]The humidifier 23 is connected to an exhaust pipe 32 through which
cathode off-gas of the fuel cell 20 is discharged. The exhaust pipe 32 is
provided with a pressure-regulating valve 33 and connected at the
downstream end thereof to the diluter 24 as viewed in the direction of
cathode off-gas flow. The fuel cell 20 has a hydrogen outlet port (not
shown) to which one end of a purge gas pipe 34 is connected. The other
end of the purge gas pipe 34 is connected to the diluter 24. The purge
gas pipe 34 is provided with a purge valve 35.

[0020]The fuel cell system 12 further includes an exhaust pipe 36 one end
36A of which is connected to the diluter 24. The exhaust pipe 36 extends
through an opening 11A of the casing 11 to the outside of the casing 11
and is opened at the other end 36B to the atmosphere. Anode purge gas
discharged from the hydrogen outlet port of the fuel cell 20 is
introduced through the purge gas pipe 34 into the diluter 24. Then the
anode purge gas is diluted in the diluter 24 with the cathode off-gas
from the exhaust pipe 32 and discharged from the exhaust pipe 36 into the
atmosphere as exhaust gas.

[0021]The fuel cell 20 is connected to a heat medium passage 37 provided
with the radiator 25. The heat medium passage 37 is thermally coupled to
the fuel cell 20. The heat medium passage 37 is provided with a
circulation pump 38 for circulating cooling water (heat medium) through
the heat medium passage 37 and the fuel cell 20. The heat medium passage
37 is provided with a temperature sensor 39 for measuring the temperature
of cooling water circulating through the heat medium passage 37. The
radiator fan 26 is provided between the fuel cell 20 and the radiator 25.
The radiator fan 26 produces airflow toward the radiator 25 by drawing
outside air, thereby increasing airflow through the radiator 25. In FIG.
2, the symbol Y denotes the airflow generated in the casing 11 by the
rotation of the radiator fan 26 and directed from the fuel cell 20 toward
the radiator 25 and further toward the other end 36B of the exhaust pipe
36.

[0022]There is provided a guide pipe 40 (guide) in the casing 11. Airflow
from the radiator fan 26 through the radiator 25 is guided through the
guide pipe 40 and directed out of the casing 11. The guide pipe 40 has at
one end thereof a tapered portion 40A the diameter of which is gradually
increased toward the radiator 25. The guide pipe 40 has at the other end
an outlet 40B projecting out of the casing 11 from the opening 11A. The
outlet 40B of the guide pipe 40 is located adjacent to the other end 36B
of the exhaust pipe 36 so as to diffuse exhaust gas discharged from the
exhaust pipe 36. In FIG. 2, the airflow passing through the guide pipe 40
is indicated by arrow Y.

[0023]The radiator fan 26, the pressure-regulating valve 33, the purge
valve 35, the circulation pump 38 and the temperature sensor 39 are all
connected to the ECU 27. The power control unit 19 for converting the
electric power generated in the fuel cell 20 into the electric power for
a base vehicle is connected to the fuel cell 20. The fuel cell 20 is also
connected to a capacitor 41 as an auxiliary power supply for the fuel
cell 20 that is used when starting the forklift truck.

[0024]The following will describe the operation of the fuel cell system 12
with reference to FIGS. 2 through 4.

[0025]While the fuel cell 20 is in operation, hydrogen, the pressure of
which has been regulated by the pressure-regulating valve, is supplied
from the hydrogen tank 21 through the pipe 28 to the anode of the fuel
cell 20. Compressed air supplied from the air compressor 22 to the
humidifier 23 is humidified and supplied through the pipe 30 to the
cathode of the fuel cell 20. The hydrogen supplied to the anode is
decomposed into hydrogen ions and electrons in the presence of a
catalyst, and the hydrogen ion moves through the electrolyte membrane to
the cathode with water. In the cathode, oxygen in the air is reacted with
the hydrogen ion having passed through the electrolyte membrane and the
electrons having passed through an external circuit, so that water is
produced. The water thus produced in the cathode, which is in the form of
a vapor, is discharged with unreacted air through the pipe 31 into the
humidifier 23 as cathode off-gas and delivered through the exhaust pipe
32 to the diluter 24.

[0026]As shown in FIG. 3, during a given period of time t1 (first time)
after the start of the fuel cell 20, the radiator fan 26 is operated at a
high speed regardless of the cooling water temperature under the control
of the ECU 27. In the embodiment, the time t1 is shorter than the time t0
(second time) after which normal controlling of the radiator fan 26 based
on the cooling water temperature is started, and the radiator fan 26 is
operated at the maximum rotation speed Rm within the speed control range
during the time t1 after the start of the fuel cell 20.

[0027]When the fuel cell 20 is started after an interval of time in hours
or days, anode purge is carried out because hydrogen concentration in the
anode is then low due to the diffusion of nitrogen from cathode to anode
and the efficiency of electric power generation of the fuel cell 20 is
low, accordingly. Specifically, the purge valve 35 is opened and closed
intermittently under the control of the ECU 27 so that hydrogen gas is
sufficiently supplied to the anode and water and nitrogen collected at
the anode is discharged with the hydrogen gas into the purge gas pipe 34.
In the anode purge during the time t1 after the start of the fuel cell
20, as shown in FIG. 3, the purge valve 35 is operated so that opening of
the purge valve 35 for time t3 and closing of the purge valve 35 for time
t4 are repeated alternately.

[0028]As shown in FIG. 3, hydrogen concentration in the anode is increased
gradually from low to high by the above intermittent operation of the
purge valve 35. In the drawing, the symbol t2 denotes the time it takes
for the hydrogen concentration in the anode to increase from low to high.
The efficiency of electric power generation of the fuel cell 20 is
increased as the hydrogen concentration in the anode is increased. When
the hydrogen concentration in the anode is high, the efficiency of
electric power generation of the fuel cell 20 is the highest and the
output of the fuel cell 20 is the highest. As shown in FIG. 3, the
electric power generation by the fuel cell 20 becomes the maximum when
the time t2 has elapsed after the fuel cell 20 is started, which allows
electric power supply to almost all of the loads of the forklift truck.
When the time t4 in the anode purge becomes shorter, that is, when the
duty ratio (t3/t4) of the operation of the purge valve 35 becomes larger,
the hydrogen concentration in the anode reaches high level rapidly, which
shortens the time t2 it takes for the output of the fuel cell 20 to reach
the maximum. In the present embodiment, the time t1 is substantially
equal to the time t2.

[0029]On the other hand, the anode purge gas discharged into the purge gas
pipe 34 is supplied to the diluter 24, diluted with the cathode off-gas
from the exhaust pipe 32, and then discharged through the exhaust pipe 36
out of the casing 11 as exhaust gas. In this case, airflow produced by
the radiator fan 26 and flowed through the radiator 25 is guided by the
tapered end 40A of the guide pipe 40 and directed toward the outlet 40B
in the guide pipe 40. The airflow delivered out of the guide pipe 40 is
blown against the exhaust gas discharged out of the exhaust pipe 36 that
is located adjacent to the outlet 40B of the guide pipe 40, so that the
exhaust gas is forcedly diffused into the atmosphere.

[0030]During the time t1 after the start of the fuel cell 20, the radiator
fan 26 is operated at the maximum rotation speed Rm, and the flow rate of
air blown against the exhaust gas is the highest, accordingly. This
allows the exhaust gas to be diffused forcedly by such increased air flow
rate and also the hydrogen concentration in the exhaust gas to be
decreased below a predetermined level (e.g., 2% or less in UL standard),
even in a condition that hydrogen concentration in the exhaust gas
discharged from the exhaust pipe 36 is high. Thus, the time it takes for
the hydrogen concentration in the anode to become high, that is, the time
t2 it takes for the fuel cell output to become the maximum can be
shortened by increasing the duty ratio of the operation of the purge
valve 35 so as to increase frequency of the anode purge.

[0031]Electric power generation by the fuel cell 20 is started
concurrently with the start of the fuel cell 20. Fuel cell temperature is
gradually increased due to the heat produced with the electric power
generation. The fuel cell 20 has an appropriate temperature at which the
efficiency of electric power generation is the highest. To prevent
excessive increase of the temperature of the fuel cell 20 and thereby to
maintain the fuel cell 20 at the appropriate temperature, cooling water
is circulated between the radiator 25 and the fuel cell 20 through the
heat medium passage 37 so that the heat generated in the fuel cell 20 is
transferred to the radiator 25 and radiated from the radiator fins into
the atmosphere. Such heat radiation from the fuel cell 20 depends on the
flow rate of cooling water circulating through the heat medium passage 37
and also on the rotation speed of the radiator fan 26. The higher the
flow rate of cooling water and the rotation speed of the radiator fan 26
become, the larger the mount of heat radiation becomes.

[0032]As shown in FIG. 3, after the fuel cell 20 is started, the
temperature of the cooling water circulating through the heat medium
passage 37 is gradually increased as the time elapses. In the present
embodiment, during the time t1 after the start of the fuel cell 20, the
circulation pump 38 is operated under the control of the ECU 27 so that
the flow rate of the cooling water circulating through the heat medium
passage 37 is lower than a flow rate when the hydrogen concentration in
the anode is high, the efficiency of electric power generation of the
fuel cell 20 is the highest and the output of the fuel cell 20 is
maximum. Specifically, during the time t1 after the start of the fuel
cell 20, the radiator fan 26 and the circulation pump 38 are controlled
by the ECU 27 so that the radiator fan 26 is operated at the maximum
speed Rm and the flow rate of the cooling water circulating through the
heat medium passage 37 is at a level that is enough to reduce the
localized concentration of the heat generated in the fuel cell 20. Thus,
during the time t1 after the start of the fuel cell 20, the radiator fan
26 is operated at the maximum rotation speed Rm while the flow rate of
the cooling water circulating through the heat medium passage 37 is low,
which prevents excessive cooling of the fuel cell 20 and also prevents
reduced efficiency of electric power generation due to a decrease of the
fuel cell temperature at the start of the fuel cell 20. As a result, it
takes less time for the fuel cell 20 to reach an appropriate temperature
range in which the electric power generation is performed with a high
efficiency.

[0033]As shown in FIG. 3, after the time 11 has elapsed, the radiator fan
26 is stopped, and the rotation speed of the circulation pump 38 is
increased so that the flow rate of cooling water is changed from low to
middle. The circulation of cooling water at the middle flow rate is
continued until the time t0 has elapsed, and the cooling water
temperature, which is detected by the temperature sensor 39 located
outside the fuel cell 20, is increased approximately linearly by the
circulation of cooling water at the low and middle flow rate. When the
time t0 has elapsed and the cooling water temperature reaches a given
cooling starting temperature T1, controlling of the radiator fan 26 based
on the cooling water temperature is started, and the radiator 25 then
performs its normal radiating function so as to prevent an increase of
the cooling water temperature. After the time t0 has elapsed, the flow
rate of cooling water is increased gradually as the cooling water
temperature is increased. The rotation speed of the radiator fan 26 is
controlled based on the previously programmed relation between the
cooling water temperature and the rotation speed of the radiator fan 26.
This allows the fuel cell 20 to operate in an appropriate temperature
range in which electric power generation is done with the highest
efficiency.

[0034]FIG. 4 shows graphs as an example for comparison with the embodiment
of the present invention. In the case of FIG. 4, the high-speed operation
of the radiator fan 26 and the circulation of the cooling water through
the heat medium passage 37 at low flow rate as in the case of FIG. 3 are
not performed. During the time t0 after the start of the fuel cell, the
radiator fan 26 is at a stop. When the time t0 has elapsed and the
cooling water temperature reaches the cooling starting temperature T1,
controlling of the radiator fan 26 based on the cooling water temperature
is started. In this case, the cooling water is circulated at middle flow
rate until the time t0 elapses after the start of the fuel cell. After
the time t0 has elapsed, the flow rate of cooling water is gradually
increased as the cooling water temperature is increased.

[0035]During the time t0 after the start of the fuel cell 20, the radiator
fan 26 is at a stop, and the amount of airflow blown against the exhaust
gas discharged from the exhaust pipe 36 is approximately zero,
accordingly. This makes it impossible to diffuse the exhaust gas forcedly
by airflow and also to decrease the hydrogen concentration in the exhaust
gas below a predetermined desired level (e.g. 2% or less in UL standard)
in a condition that hydrogen concentration in the exhaust gas from the
exhaust pipe 36 is high. Thus, as shown in FIG. 4, the valve closing time
t4 of the purge valve 35 needs to be increased, that is, the duty ratio
of the operation of the purge valve 35 needs to be decreased. This
increases the time it takes for the hydrogen concentration in the anode
to becomes high, resulting in increased time t5 it takes for the fuel
cell output to become the maximum.

[0036]As described above, in the fuel cell system 12 according to the
embodiment of the present invention, the operation of the radiator fan 36
and the circulation pump 38 is controlled during the time t1 after the
start of the fuel cell 20 so that the radiator fan 26 is operated at the
maximum rotation speed Rm and the flow rate of cooling water circulating
through the heat medium passage 37 is low. This allows shortening of the
time it takes for the fuel cell output to become the maximum while
preventing temperature drop at the start of the fuel cell.

[0037]The fuel cell system 12 according to the first embodiment offers the
following advantages.

(1) During the time t1 after the start of the fuel cell 20, the radiator
fan 26 is operated at the maximum rotation speed Rm, and the flow rate of
air blown against the exhaust gas is the highest, accordingly. This
allows the exhaust gas to be diffused forcedly by such increased airflow
rate and also the hydrogen concentration in the exhaust gas to be
decreased below a predetermined level, even in a condition that hydrogen
concentration in the exhaust gas discharged from the exhaust pipe 36 is
high. Thus, the time it takes for the hydrogen concentration in the anode
to become high, that is, the time t2 it takes for the fuel cell output to
become the maximum can be shortened.(2). The operation of the circulation
pump 38 is controlled during the time t1 after the start of the fuel cell
20 so that the flow rate of cooling water circulating through the heat
medium passage 37 is low. Thus, during the time t1 after the start of the
fuel cell 20, the radiator fan 26 is operated at the maximum rotation
speed Rm while the flow rate of the cooling water circulating through the
heat medium passage 37 is low, which prevents excessive cooling of the
fuel cell 20 and also prevents reduced efficiency of electric power
generation due to a decrease of the fuel cell temperature at the start of
the fuel cell 20. As a result, it takes less time for the fuel cell 20 to
reach an appropriate temperature range in which the electric power
generation is performed with a high efficiency, and the time t2 it takes
for the fuel cell output to become the maximum can be shortened
further.(3) The time t1 is shorter than the time t0 after which
controlling of the radiator fan 26 based on the cooling water temperature
for maintaining the fuel cell 20 at the appropriate temperature is
started. Thus, it is only necessary to operate the radiator fan 26 at a
high speed using the time during which the radiator fan is at a stop in a
conventional fuel cell system. Such operation of the radiator fan 26 can
be accomplished only by a minimum system modification such as a minor
change to the program in the ECU 27 of the fuel cell system 12.(4) Anode
purge gas from the fuel cell 20 is diluted in the diluter 24 before being
discharged from the exhaust pipe 36, which reliably decreases hydrogen
concentration in the exhaust gas below a predetermined level.(5) The
forklift truck 10 equipped with the fuel cell system 12 of the present
embodiment prevents accumulation of anode purge gas in a work area such
as warehouse where the forklift truck 10 is used.

[0038]The above embodiment may be modified in various ways as exemplified
below.

[0039]Although in the previous embodiment the radiator fan 26 is operated
at the maximum rotation speed Rm during the time t1, the rotation speed
during the time t1 may be 50% or more of the maximum rotation speed Rm.

[0040]Although in the previous embodiment the circulation pump 38 is
operated so that the flow rate of cooling water is low during the time t1
after the start of the fuel cell, the circulation pump 38 may be at a
stop so that the flow rate is approximately zero. This further prevents a
temperature drop of the fuel cell at its start, thus allowing further
shortening of the time it takes for the fuel cell output to become the
maximum.

[0041]Although in the previous embodiment anode purge gas is diluted in
the diluter 24 before being discharged into the atmosphere, the anode
purge gas may be discharged through the purge gas pipe 34 directly into
the atmosphere without being diluted. In such a case, it is so arranged
that the outlet of the purge gas pipe 34 as the exhaust pipe is located
adjacent to the outlet 40B of the guide pipe 40.

[0042]In the previous embodiment, airflow from the radiator fan 26 through
the guide pipe 40 is blown against the anode purge gas (exhaust gas)
discharged from the exhaust pipe 36. Alternatively, it may be so arranged
that the anode purge gas discharged from the exhaust pipe 36 is blown
against the airflow from the radiator fan 26 through the guide pipe 40.

[0043]In the previous embodiment, the radiator fan 26 is located between
the fuel cell 20 and the radiator 25. The radiator fan 26 may be located
anywhere as long as the radiator fan 26 functions to increase the flow
rate of air through the radiator 25. For example, the radiator fan 26 may
be located on the opposite side of the fuel cell 20 from the radiator 25,
or on the opposite side of the radiator 25 from the fuel cell 20.

[0044]The radiator fan 26 may be used not only for cooling the radiator 25
but also for cooling other components in the fuel cell system 12.

[0045]In the previous embodiment, the airflow from the radiator fan 26
through the radiator 25 is guided by the guide pipe 40 and blown against
the anode purge gas discharged from the exhaust pipe 36. Alternatively,
it may be so arranged that part of the airflow from the radiator fan 26
which has not passed through the radiator 25 is guided by the guide pipe
40 and blown against the anode purge gas discharged from the exhaust pipe
36.

[0046]In the previous embodiment, the radiator fan 26 is operated at the
maximum rotation speed Rm and the circulation pump 38 is operated at low
flow rate during the time t1 after the start of the fuel cell.
Alternatively, it may be so controlled that the operation of the radiator
fan 26 at the maximum rotation speed Rm and the operation of the
circulation pump 38 at low flow rate is performed in synchronization with
the anode purge operation. Specifically, as shown in FIG. 5, the radiator
fan 26 may be operated at the maximum rotation speed Rm during the valve
opening time t3 of the purge valve 35, and the radiator fan 26 may be
kept at a stop during the valve closing time t4 of the purge valve 35.
The circulation pump 38 may be operated at low flow rate during the valve
opening time t3 and operated at middle flow rate during the valve closing
time t4.

[0047]The present invention may be applied not only to a forklift truck
but also to other types of industrial vehicles such as a construction
vehicle and a towing tractor, or to general-purpose vehicles such as a
passenger car and a bus.

[0048]In the present embodiment, the operation of the radiator fan 26 at
high speed and the operation of the circulation pump 38 at low flow rate
are performed in synchronization with the anode purge operation at the
start of the fuel cell. Alternatively, such operation of the radiator fan
26 and the circulation pump 38 may be performed in synchronization with
the anode purge operation while the fuel cell is in operation.